This has to be my favorite experiment I’ve done so far! The Barking Dog is a really exciting demonstration of a combustion reaction, or a chemical reaction where one material burns in another. This demonstration very beautifully brings together many of the other topics Bill and I talk about in the show, like energy, light, sound, and color!

In the Barking Dog reaction, a mixture of two gases is placed in a long glass tube that is sealed at both ends with rubber stoppers. When ready, the tube is stood upright, one of the stoppers is removed, and the mixture is lit at the top with a match. A wave of flame quickly moves down the tube as the gases burn, speeding up the farther down it is. The flame is bright blue because of the sulfur that is produced in the chemical reaction (sulfur burns with a blue color), and the rapidly expanding gases shoot out of the top of the tube with a loud “Woosh” that sort of sounds like a dog’s bark!

The video is a clip from our first live show, held at the STEM Community Forum on March 1st

The Science:

This reaction happens between two chemical compounds: nitrous oxide (chemical symbol: N2O) and carbon disulfide (CS2). We fill the tube with the N2O first, which is an invisible gas like air. You might know nitrous oxide better by it’s more common name: laughing gas! They use this at the dentists office as an anesthetic, something to make your body numb so you don’t feel it while they work on your teeth.

After that, we add a very tiny amount of the carbon disulfide. This is a liquid, but very quickly evaporates into a gas when it’s placed in the tube. This material is very flammable and burns easily, allowing us to light the mixture with a match or lighter. When ignited, we get a very vigorous reaction that produces a lot of heat, light, and sound as the initial chemicals are converted into different ones:
CS₂ (g) + 4N₂O (g) → CO₂ (g) + SO₂ (g) + S (s) + 4N₂ (g)
The letters in parentheses in the equation tell you what phase the chemical is in:

(s) = solid
(l) = liquid
(g) = gas

From the equation, you can see that this reaction produces elemental sulfur (chemical symbol: S)! That’s why at the end of the reaction you see the tube has a light dusting of yellow on the inside of it – that’s the sulfur that was made! The rest of the products escape as invisible gases.

Sometimes I like to play a little trick on my brother, who doesn’t know anything about chemistry, and get him to mix some chemicals together for me. After getting him to put on some safety gear (gloves and goggles), I tell him to mix two different chemicals together for me. Usually he does it wrong (on purpose probably), but that’s where the trick comes in. If you put too much of one into the other, it almost explodes with tons of foam and steam! It’s a bonus if he gets foam on him, but usually he gets out of the way in time.

Here’s a short video of me testing this reaction out.

The Science:

The liquid in the large flask is hydrogen peroxide (chemical symbol H2O2) like you can find in the drug store for cuts and scrapes, except the stuff I have is 10 times more powerful! It’s mixed with some ordinary liquid dish soap to give it that green color.

You may have heard water called “H2O” before, and if you have then you already know a bit about chemistry! That tells you that water is made up of 2 parts hydrogen and 1 part oxygen, both gases found in the Earth’s atmosphere. Hydrogen peroxide is a lot like water, as you can see from its chemical symbol H2O2. It’s basically water with another oxygen atom attached to it. That oxygen atom would be much happier if it were a gas rather than stuck onto a water molecule, so it’s easy to break peroxide apart back into water and oxygen.

That’s exactly what I do here. Peroxide breaks down on its own over time, but we can speed it up a lot by adding in another chemical. I’m using potassium iodide (symbol KI) dissolved in water. This greatly speeds up (catalyzes) the peroxide breaking up, which is called a decomposition reaction:

2H₂O₂ → 2H₂O + O₂

The oxygen gas (O₂) that’s produced blows a lot of small bubbles in the dish soap in solution, which creates a huge amount of foam! The reaction also releases a lot of heat, which is why the foam becomes hot and steams in the air.

One of the demonstrations we do in our show is the classic chemistry experiment called the Chemical Chameleon. This is a color changing reaction that proceeds on its own through a number of different beautiful colors, and involves some really interesting chemistry.

The demonstration is done by preparing two solutions.

	Solution A: About 2mg potassium permanganate (chemical symbol: KMnO4) is dissolved in 50mL of distilled water. We only need a tiny amount of this, because it creates a very intense purple color in solution and can be too dark to see if too much is used.
	Solution B: 6g of sugar (C12H22O11) and 10g of sodium hydroxide (NaOH, also known as Lye) is dissolved in about 750mL of distilled water.

Note: It’s always important to use distilled water in any chemistry experiment, because we want to make sure to avoid any sources of contamination. Tap water has lots of other things in it that are good for you, but might be bad for a chemical reaction.

Simply pouring solution A into solution B gets things going! For best results, we swirl the flask to get everything well mixed. Immediately, the deep purple color of solution A changes to blue, and very quickly after that turns green. Then, much more slowly, over a few minutes, the green fades into a yellow-orange. This is actually caused by tiny solid particles of a new chemical, manganese dioxide (MnO2) that’s been formed during the reaction. If allowed to sit long enough, these will settle to the bottom and the color of the liquid will turn clear again!

Here’s a video of the experiment in action!

(Note: This is test footage that will be replaced with a nicer demo soon.)

The Science:

Even though this experiment is easy to perform, there’s actually some really complicated and interesting chemistry going on! It involves something called a redox reaction. This basically means that new compounds are formed when one chemical takes electrons from another chemical. Here, the potassium permanganate is reduced, meaning it gains electrons, and the sugar is oxidized, meaning it loses some.

This happens in two steps. In the first step, the permanganate ion (the part of the potassium permanganate that changes) is reduced to the manganate ion:

MnO₄^– + e^– → MnO₄^-2

The compound on the left is purple, and the one on the right is green. As this reaction is going, there is some purple and some green in the solution and these combine to make it look blue at the beginning.

Next,  the green manganate is reduced again into manganese dioxide:

MnO₄^-2 + 2H₂O + 2e^– → MnO₂ + 4OH^–

The manganese dioxide is a brown solid, but it’s in such tiny particles that it appears to make the liquid turn yellow.

So, that’s how you make colors with chemistry!

Disposal:

I’ve gotten several requests in the comments to talk about disposal, so I thought I’d mention it here. Proper disposal is an extremely important part of any chemistry experiment, to keep yourself and others safe and to protect the environment. In this experiment, the sugar is reduced to a different type of sugar (which is nonhazardous), and the potassium permanganate becomes manganese dioxide (MnO₂, which is also harmless). The only thing we need to worry about is the NaOH, and this is easily neutralized by the addition of some acid until a pH of 7 is reached. I usually use 9.5M hydrochloric acid (standard concentration of hardware-store grade acid), which means we need about 26mL of acid to fully neutralize the solution. The resulting solution of sugary, salty water is then perfectly safe to pour down the drain.

The Tesla Coil we use in our show is a modern version of a classic device invented by Nikola Tesla. Our coil is called a DDSSTC (Dual Resonant Solid State Tesla Coil). This means the coil is controlled by transistors, just like in the computer you are reading this on. This is a modern upgrade to the device Tesla created, called the SSTC (Spark Gap Tesla Coil). But how does it work?

Nikola Tesla is not a well known man, but he is the father of our electric age. He was the pioneer of Alternating Current (AC) the type of electricity that comes out of your wall socket. This is different from Direct Current (DC) the type of electricity that comes from a battery. Tesla knew that AC was a much better way to deliver electricity to your house, and he helped design the electrical grid we use today.

Though Tesla didn’t like they way the world would look with so many wires running about to deliver power. He started work on a way to deliver power wirelessly so that there would be no need to run so many wires around the planet. His work lead him to the invention of the Tesla Coil.

A Tesla Coil works much in the same way your cell phone charger works, but in reverse. Instead of reducing voltage, it increases it. This is called a transformer. It has two main sets of windings, a primary winding and a secondary winding. The difference in the number of turns in each winding determines if the voltage out will be higher or lower then the voltage in and by how much. A Tesla Coil has very few turns on the primary (input) side, and many turns on the secondary (output) side. This increases the voltage tremendously.

But a Tesla Coil does much more then increase voltage, it increases frequency.  You can see a schematic of a Spark Gap Tesla Coil above. The capacitor and spark gap work together to create a very high frequency pulse to drive the primary coil. Tesla designed it this way because once you create very high voltage electricity at a very high frequency, you can create a field of energy with which to transmit power.

Here is a video of our Tesla Coil transmitting enough power to light up a Florescent light bulb.

Our Tesla Coil is different the the schematic above in that it has no spark gap. Instead, a series of transistors ‘switch’ current on and off at a very high frequency. This means we can control the Tesla Coil at different frequencies, and even produce sounds.

Here’s a video of two Dual Resonant Solid State Tesla Coils playing a song.

So this is how a Tesla Coil works. It creates high-voltage electricity at a high frequency. And when you produce this kind of electricity, you can do cool things like power light-bulbs with no wires.